REPTILES 



205 



jaws is separated by an ill-marked neck from a long body, and 

 this again passes into a long and powerful tail, which is the 

 chief swimming organ. The body is flattened from above 

 downwards, and the tail, in correspondence with its function, 

 from side to side. The sprawling lizard-like limbs do not lift 

 the trunk well off the ground, the feet are webbed to some 

 extent, and there are five digits in the fore- but only four in the 

 hind-limb, the little toe being represented only by a small bone 

 visible in the prepared skeleton. 



Returning to the head, we find several arrangements that 

 are of special use to an aquatic form. The slit-like nostrils are 

 situated on the tip of the snout, and can be closed by muscular 

 action so as to prevent entry of water when the animal is sub- 

 merged, while their position enables breathing to go on when 

 only a very small part of the body is above the surface. As in 

 birds and lizards (as well as some mammals), the eye possesses 

 not only upper and lower eyelids but also a third eyelid which 

 can be drawn over it for protection, and the tympanic membrane, 

 corresponding in position to that of the lizard (see p. 192), is 

 provided with a little protective flap or " ear-lid " which can be 

 folded over it. 



The chief peculiarity of the skin (fig. 133) is found in the 

 fact that it develops a very complete exoskeleton, composed 

 externally of rectangular horny scales, which on the tail and 

 upper and lower sides of the trunk are disposed in regular 

 longitudinal and transverse rows. Some of these scales project 

 upwards from the upper side of the tail, forming a jagged crest. 

 This, however, is not all. Underlying the scales of the upper 

 and sometimes of the under surface are sculptured bony plates 

 (scutes), which are developed in the deeper part of the skin 

 (dermis). The arrangement of the scales and scutes differs in 

 different species, and affords characters which are of use in 

 classification. The strong musky odour which distinguishes 

 crocodiles is due to the secretion of musk-glands, some of which 

 open on the edge of the upper jaw, others into the cloaca, and 

 the remainder on the dorsal surface of the trunk. 



The endoskeleton of the crocodile can be compared point 

 for point with that of the lizard, though it presents a number 

 of interesting peculiarities. In the large and massive skull 

 the boundaries between the separate bones can be always 



